We have previously shown that a deficiency of pterin carbinolamine dehydratase (PCD), an enzyme that functions as part of the system that catalyzes the conversion of phenylalanine to tyrosine, can cause hyperphenylalaninemia (HPA). Because PCD is only enriched in relatively inaccessible tissues such as kidney and liver, detection of PCD deficiency is difficult. We have shown, however, that mRNA for this enzyme is easily detectable in human hair follicles and in white blood cells. The detection of mRNA for PCD in hair follicles and white blood cells will greatly facilitate diagnosis of HPA due to a lack of PCD and should serve as a useful guideline for initiation of treatment of this disease. We have been studying the properties of two different mutant forms of human phenylalanine hydroxylase that are associated with phenylketonuria (PKU) in vivo, I65T, in which an isoleucine residue at position 65 is replaced by a threonine residue and T92I, in which a threonine at position 92 is replaced by an isoleucine. Both mutant proteins have been purified and their catalytic characteristics determined, along with the wild- type enzyme. Although significant differences in the Km values for phenylalanine and tetrahydrobiopterin have been detected in the mutant proteins, these changes do not appear to be sufficient to cause PKU in the patients. The most likely explanation, therefore, for how these mutations lead to the PKU phenotype is that in vivo they are degraded more rapidly than the wild-type enzyme. We plan to investigate whether this explanation is valid.